Aspects of leukocyte physiology to be studied: A. The lymphatic system: Lymphocytes from hematologically normal and abnormal human sources are obtained from marrow, blood, lymph nodes, spleen, and thymus. Thoracic duct cells are also available in some instances. They are categorized into T and B subpopulations on the basis of 1) Blastogenesis to non-specific mitogens, specific delayed hypersensitivity antigens, and histocompatibility (HLA) antigens; 2) Surface Ig by immunofluorescent and rosette forming techniques; 3) Lymphotoxin or lymphotoxin-like activity; 4) Influence on monocyte- macrophage transition and function. B. Monocyte and macrophage function from normal and abnormal human sources are assessed with respect to behavior in culture, response alterations by lymphocytes and antigen, bacterial killing, in selected instances, elaboration of colony stimulating factor (CSF) and response to endotoxin. C. Granulocytes from similar sources are assessed with respect to chromosomal markers and bacterial killing in selected instances, capacity to release CSF in conditioned medium and feeder layer systems. D. Hematopoiesis in normal and abnormal conditions is assessed by the usual criteria plus colony forming capacity (for production of mixed granulocytic and monocytic colonies) under standardized conditions. A reproducible assay is applied to aspirated human marrow. The correlation of these findings with the clinical and histological picture will permit better functional comprehension of the leukemias and lymphomatous diseases of man and, hopefully, to more enlightened treatment.